1. Field of the Invention
The present invention relates to a scroll type compressor provided with a fixed scroll and an orbiting scroll. More particularly, it relates to an improved scroll tip and discharge port arrangement.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 59-218380 discloses a compressor as shown in FIGS. 13 to 15. This compressor has a fixed scroll 91 fixed in a housing 90 and an orbiting scroll 92. The orbiting scroll 92 is supported revolvable around the axis of the fixed scroll 91 in the housing 90.
The fixed scroll 91 comprises a fixed end plate 911 and a fixed spiral element 912 formed integrally with the bottom surface of the fixed end plate 911. The fixed spiral element 912 has its inner and outer walls formed along involute curves. Likewise, the orbiting scroll 92 comprises an orbiting end plate 921 and an orbiting spiral element 922 formed integrally with the top surface of the orbiting end plate 921. The orbiting spiral element 922 also has its inner and outer walls formed along involute curves. The fixed spiral element 912 and the orbiting spiral element 922 slide against each other.
In this compressor, a drive shaft 95 rotates by the interaction of a stator 93 and a rotor 94 mounted on the drive shaft 95. As the drive shaft 95 rotates, the orbiting scroll 92 revolves around the axis of the fixed scroll 91 by the work of an eccentric pin 95a slightly eccentric to the drive shaft 95 and a rotation preventing device 96. In accordance with this revolution, a plurality of compression chambers 97 to be formed in a sealed state between the fixed scroll 91 and the orbiting scroll 92 move toward the center of the fixed scroll 91 while sequentially reducing their volumes.
A discharge port 98 is provided in the center of the fixed end plate 911. As shown in FIGS. 13 and 14, the fully compressed gas in a compression chamber 971 is discharged through the discharge port 98 into a discharge chamber 99. As the orbiting scroll 92 revolves, the fluid in the next compression chamber 972 (which follows the compression chamber 971) is sequentially discharged from the discharge port 98.
As shown in FIGS. 14 and 15, a tapered surface 922b is cut in a tip portion 922a in the center of the orbiting spiral element 922. This tapered surface 922b and the inner wall of a center tip portion 912a of the fixed spiral element 912 constitute a passage that permits communication between the compression chamber 971 in the final compression stage and the discharge port 98. The existence of this passage reduces the discharge resistance at the time the gas in the compression chamber 971 is discharged through the discharge port 98 into the discharge chamber 99.
In the conventional compressor, the end portions of the fixed spiral element 912 and orbiting spiral element 922 slide against the end plates of the mating scrolls while being pressed together in order to form sealed compression chambers. Both tip portions 912a and 922a receive the pressure of the gas in the most compressed state at the final compression state. Those tip portions 912a and 922a should therefore have a sufficient strength.
The formation of the tapered surface 922b at the end position of the tip portion 922a however decreases the strength of the tip portion 922a significantly. The tip portion 922a may therefore be damaged by the sliding action against the tip portion 912a and the high pressure. Because of these drawbacks, it is very difficult to use this type of tip design in a scroll type compressors for vehicles, which is required to operate under the conditions of fast rotation and high compression.
Further, in the conventional compressor, the compressed gas in the compression chamber 971 is discharged to the discharge port 98, passing through an opening enclosed by the circular inner wall of the discharge chamber 98 and the curved inner wall or the tip portion 922a of the orbiting spiral element 922. As the orbiting scroll 92 revolves, the tip portion 922a of the orbiting spiral element 922 gradually reduces the cross-sectional area of the passage between the discharge port 98 and the compression chamber 971.
Immediately before completion of the gas discharging, the cross-sectional area of the passage between the discharge port 98 and the compression chamber 971 decreases rapidly. Even if the tapered surface 922b is provided at the tip portion 922a, the discharge resistance will not be reduced sufficiently immediately before completion of the gas discharging when such reduction is needed most.
Furthermore, to optimize compression efficiency, it is desirable that the following compression chamber 972 does not communicate with the discharge port simultaneously with the compression chamber 971. This is because the compressed gases exiting compression chamber 971 would expand into the following chamber. The re-expansion reduces the compression efficiency.